22nd May 2025

You are asked to review an 84-year-old gentleman by nursing staff as the laboratory has rang the ward demonstrating one of your patients has a serum sodium of 123 mmol/L.

You are asked to review an 84-year-old gentleman by nursing staff as the laboratory has rang the ward  demonstrating one of your patients has a serum sodium of 123 mmol/L. His serum osmolality is reduced at 250 mOsmol/Kg.  This patient was admitted 2 weeks earlier with a new diagnosis of small cell lung cancer. On assessment he is noted to be euvolaemic and is clinically stable.

Which of the following would be the most appropriate next investigation to elicit the cause this hyponatraemia? Please select only ONE answer.